You just need add two things - the wheel-parameter definer :
this.add_wheel('wfl', wheelparam);
this.add_wheel('wfr', wheelparam);
this.add_wheel('wbl', wheelparam);
this.add_wheel('wbr', wheelparam);
this.add_wheel('wmfl', wheelparam); // the two sets of middle wheels
this.add_wheel('wmfr', wheelparam);
this.add_wheel('wmbl', wheelparam);
this.add_wheel('wmbr', wheelparam);
and the force-defining one :
steering *= 0.6;
this.steer(0, steering);
this.steer(1, steering);
if(this.started>1)
this.wheel_force(0, engine);
this.wheel_force(1, engine);
this.wheel_force(2, engine);
this.wheel_force(3, engine);
this.wheel_force(4, engine);
this.wheel_force(5, engine);
this.wheel_force(6, engine);
this.wheel_force(7, engine);
mind that the force-ones have the wheel number (0 to 7) in the exact order, as you define the wheel-parameter, ones. So you better keep the look-out on witch you take here, if you dont want all be the force-driwen and the same is for the steering -- if you want the other wheels to rotate too, and you can define the rotation on a reverted, or a number-magnified value, like this:
steering *= 0.6;
this.steer(0, steering);
this.steer(1, steering);
var steeringmf = steering * 0.8; // - its better to use another variable, cause you would probably use some other function dependend on "steering" later and you just change it by the " steering *= 0.6;" for all the rest of the script.
this.steer(2, steeringmf);
this.steer(3, steeringmf);
var steeringb = - ( steering * 0.7 );
this.steer(6, steeringb);
this.steer(7, steeringb);
Hope this helps a bit whyte the coding problem.